1. Field of the Invention
The present invention relates to a semiconductor device having a fuse used in a laser trimming, and more specifically to a semiconductor device having a fuse of the laser make-link programming type which causes an electrical short-circuit between two conductor films.
2. Description of Related Art
In the prior art, when a semiconductor memory such as DRAM (dynamic random access memory) having a large memory capacity is manufactured, a redundant circuit used for substituting normal parts for faulty parts generated in the manufacturing process is previously formed in the semiconductor memory. The redundant circuit includes fuses used for the substitution after manufactured. In addition, in a field programmable gate array (FPGA), in order to give a user's programmability in connection with function and operation, programmable fuses are provided. In the prior art, as this fuse, there is used a construction of breaking or cutting the conductor in a conductive condition (break-link), or another construction of short-circuiting a pair of conductors in an open-circuit condition (make-link).
Referring to FIGS. 1A and 1B, there are shown a plan view of one example of the prior art make-link type fuse, and a section view taken along the line C--C in FIG. 1A, which is disclosed in Japanese Patent Application Pre-examination Publication No. JP-A-61-93643, the content of which is incorporated by reference in its entirety into this application. This JP-A-61-93643 claims Convention Priorities based in two U.S. patent applications, from which there have now issued U.S. Pat. Nos. 4,665,295, 4,751,197 and 4,912,066, the content of which are incorporated by reference in its entirety into this application. In the shown example, a fuse pad 13 of polysilicon is formed on a field oxide film 12 formed on a semiconductor substrate 11, and a thermal oxidation film 14 having a thickness of about 2000 .ANG. is coated on the fuse pad 13. Conductor strips 15A and 15B of polysilicon are formed on the thermal oxidation coating film 14 and the field oxide film 12, separately from each other by a gap positioned above the fuse pad 13. Furthermore, a relatively thick silicon dioxide or phosphosilicate glass insulator film 16 is formed to over the conductor strips 15A and 15B, the coating film 14 and the field oxide film 12.
In order to short-circuit this fuse by a laser beam, a laser beam LB is focused on the fuse pad 13, as shown in FIG. 1B. As a result, the laser beam LB first removes the insulator film 16 hit with an incident laser beam spot by encroachment, as shown in FIG. 1C, and then, melts the conductive material of the end of the conductor strips 15A and 15B and the fuse pad 13. The molten conductive material of the conductor strips 15A and 15B and the fuse pad flows onto a side surface of a hole formed in the coating film 14, with the result that the conductor strips 15A and 15B are electrically connected to each other through the fuse pad 13, as shown in FIG. 1C.
Incidentally, in order to shorten a laser application time required for the make-link, it is some case that only an uppermost level insulator film on the fuse is removed by a mask used for photolithography and an etching process. In addition, in order to set the condition for the laser beam on the a product chip, a unique dummy pattern 20 having the shape of a cross as shown in FIG. 2 is formed of the same material as that of the fuse, and the laser beam is irradiated onto the crossed portion of the pattern 20. On the basis of a break condition, the laser beam used at a trimming is adjusted. Furthermore, when fuses are arranged in the form of an array, portions to be trimmed are located in a vertical or horizontal straight line.
The following problems have been encountered in the above mentioned prior art fuse.
A first problem is that many steps are required for forming the fuse. Namely, only in order to form the fuse, a conventional process is added with the step for forming the fuse pad 13, and the step for forming the coating film 14 for electrically separating the fuse pad 13 from the two conductor strips 15A and 15B.
A second problem is that after the fuse is trimmed by the laser beam, the wiring conductor is eroded with moisture or another. The reason for this is that, after the laser beam was irradiated onto the fuse, since the relatively thick insulator layer of silicon dioxide on the fuse has been removed by the encroachment due to the laser beam or by the etching process, the fuse is exposed.
A third problem is that when a number of fuses are arranged side by side, a large area is required. The reason for this is that, since the fuse pad has a size larger than the width of the conductor strips 15A and 15B, when a number of fuses having the same shape are arranged side by side, an interval between adjacent fuses must be increased by a difference between the size of the fuse pad and the width of the conductor strip.
A fourth problem is that, in a multilayer interconnection structure using an aluminum wiring conductor or others, the fuse formed at a lower level near to the semiconductor substrate cannot be trimmed by the laser beam with a high degree of yield. The reason for this is that because the insulator film on the fuse becomes thick in the multilayer interconnection structure, the laser beam is weakened until the laser beam reaches to the fuse, with the result that a satisfactory trimming cannot be achieved. In addition, in the case that the thickness of an insulator film under the fuse or a spacing between adjacent fuses is thinner or narrower than the thickness of the insulator film on the fuse, if the trimming is carried out by using a laser beam having energy sufficient to trim the fuse through the thick insulator film on the fuse, a fuse short-circuits with a conductor under the insulator film under the fuse or the adjacent fuse.
A fifth problem is that, when the fuse is trimmed on a product chip, a working efficiency for the laser trimming is low, since it is necessary to first trim the dummy pattern 20 having a unique shape as shown in FIG 2, and then to visually inspect the result of the trimming to determine the optimum condition of the laser trimming such as the energy of the laser beam.